An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time

OLD RIVERBED ON MARS

This spectacular image of part of a winding channel called Nanedi Valles was taken by the Mars Global Surveyor spacecraft in 1998. The channel is about 2.5 km across, and shows a variety of geologic features that strongly suggest it was carved by running water. (Courtesy of Malin Space Science Systems/JPL/NASA.)

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Changes in the Solar System